Stable Carbonium Ions Research Articles

The oxidation of alkanes to stable carbonium ions by antimony pentafluoride

AbstractAntimony pentafluoride is a unique reagent for the formation of stable carbonium ions by oxidation of saturated hydrocarbons. The kinetic parameters for the abstraction of hydride ion from ethane and neopentane have been determined. Compared to the reactions with mixed Brönsted‐Lewis acid systems (HF‐SbF5, molar ratio ⩾5/1, and FSO3H‐SbF5, molar ratio 1/1), the analogous reactions with pure SbF5 show considerably enhanced reaction rates. Moreover, neat SbF5 is capable of removing the aldehydic proton from aldehydes to yield oxocarbonium ions. It is concluded that for hydride abstraction in the so‐called “magic acid” (FSO3H‐SbF5, molar ratio 1/1) a mechanism involving protonation to alkanonium ions is not the main reaction pathway.

Determination of carbonium ion stabilities by ion cyclotron resonance spectroscopy

Determination of carbonium ion stabilities by ion cyclotron resonance spectroscopy

Visible absorption spectroscopy of ions derived from indene

The propagating carbonium ion formed by the reaction of indene with Lewis acids is shown to undergo rapid conversion to a more stable carbonium ion. Spectroscopic studies have shown that the latter ion has the same chromophore as the carbonium ion formed by the abstraction of hydride anion from 2-α-indanyl indene. A mechanism is proposed for the reaction.

Molecular orbital calculations of the catalytic effect of lysozyme: 1. Glu 35 as general acid catalyst

In a sequence of molecular orbital calculations, employing the iterative extended Hückel theory, we have studied a simple molecular model for key aspects of the catalytic process at the active site of hen egg white lysozyme, in its hydrolytic cleavage of a bacterial cell wall mucopolysaccharide. The sequence of calculations was designed to simulate “snapshots” of the interacting groups of atoms, along a reaction pathway proposed by others on the basis of such data as X-ray diffraction and model-building. For comparison with the uncatalyzed reaction, we did calculations on the same sequence without the model enzyme. The calculated activation energy of the catalyzed reaction is 0.51 eV less than for the uncatalyzed reaction, predicting a rate increase due to the enzyme by a factor of 4 × 10 8. The calculated changes in charge densities and bond densities support quite clearly the proposal that the catalytic mechanism begins with the action of the glu 35 residue as a general acid catalyst, the major factor in the rate-determining formation of a stable carbonium ion intermediate.

Reaction of tetraalkoxysilanes with stable carbonium ions

Tetramethoxy-, tetraethoxy-, and tetra-n-propoxysilanes split out an alkoxy group under the influence of such stable carbonium ions as the triphenylmethyl and tropylium carbonium ions.

Through-bond vs through-space interactions : The stabilization of carbonium ions

The solvolysis technique is suggested as an investigational tool for the possible opposing effects of through-bond and through-space orbital interactions. Existing data indicate, however, that such effects are negligible. A rationalization of the results is presented.

Photo-oxidation of triarylmethanes sensitised by carbonium ions

Absorption of light by stable carbonium ions (xanthydryl, triphenylmethyl, and 5H-dibenzo[a,d]cycloheptenyl cations) in solution in trifluoroacetic acid containing oxygen brings about oxidation of triarylmethanes to the corresponding triarylmethyl cation. Excess of molecular oxygen inhibits the photo-oxidation. The dependence of the quantum yield for photo-oxidation on the concentration of triarylmethane indicates that the hydrocarbon reacts directly with an excited state, believed to be triplet, of the sensitising cation. The effect of ring substituents in triphenylmethane on the partitioning of the triplet sensitiser between oxygen quenching and reaction with the hydrocarbon is interpreted as evidence that the photo-oxidation is initiated by electron transfer from triaylmethane to the excited sensitiser.

Stable carbonium ions. Part III. Reactions involving 1,3-diarylallyl and 1-ferrocenyl-3-arylallyl cations

Equilibrium constants have been measured for the acid-catalysed equilibration of a series of 1,3-diarylpropenols (chalcols) and 1(3)-ferrocenyl-3(1)-arylallyl ethers. The thermodynamic stability of the styryl system is increased by substitution in the aromatic ring of groups exerting either a positive or a negative resonance effect (e.g., p-OMe, p-NO2) or a positive inductive effect (e.g., p-Me). The styryl system is destabilised by the introduction of groups exerting a negative inductive effect (e.g., p-Cl, m-NO2). Conjugation of a carbon–carbon double bond with a ferrocenyl group confers greater thermodynamic stability than that provided by conjugation with an aryl group. In all of these cases, the effects are small. The rate of acid-catalysed chalcol equilibration is accelerated by electrondonating substituents and retarded by electron-withdrawing substituents in the aromatic rings. Analysis of the isomeric chalcol mixtures formed under kinetic control in the solvolysis (80% aqueous acetone) of a series of 1,3-diarylallyl p-nitrobenzoates has shown that positive charge distribution in the intermediate 1,3-diarylallyl cations is moderately sensitive to the nature of the substituents present in the aromatic rings.

Reaction of allyl and benzyl alcohols, and their toluene-p-sulphonates, with furan

2-Allyl- and 2-benzyl-furans may be prepared from allyl or benzyl alcohols which are capable of generating stable carbonium ions. The tosyl derivative of the alcohol is refluxed with furan in acetonitrile. The reaction is catalysed by lithium perchlorate. Some alcohols can react directly with furan under acidic conditions. Alcohols investigated include 2-benzylallyl alcohol (8), E-α-methylcinnamyl alcohol (2) and its p-nitro- and p-methoxy-derivatives, (1) and (3), benzyl alcohol, p-nitrobenzyl alcohol, and p-methoxybenzyl alcohol. Both geometrical isomers of α-methyl-p-nitrocinnamyl alcohol, (1) and (13), were prepared and their stereochemistry established by chemical and spectroscopic methods. Only the E-isomer (1) reacts with furan, however. The mechanism of the reaction is discussed and attempts to achieve a 1,3-dipolar cycloaddition are described.

The structure of a new type stable carbonium ion by X-ray determination of two analogous salts, [C 24H 18N] +BF 4−, [C 16H 12N] +ClO 4−

The structure of a new type stable carbonium ion by X-ray determination of two analogous salts, [C 24H 18N] +BF 4−, [C 16H 12N] +ClO 4−

Competitive reactions of nucleophiles—III : The azide probe

The azide competition factors ( k N 3 ,−/ k SOH) of cyclopentyl mesylate ( 1), 1-methylcyclopentyl chloride ( 2), cyclohexyl brosylate ( 3), benzhydryl chloride ( 4), benzhydryl bromide ( 5), 3β-cholestanyl brosylate ( 6), 3α-cholestanyl brosylate ( 7), 2-methyl-2-adamantyl chloride ( 8), 1-adamantyl bromide ( 9) and 2-adamantyl tosylate ( 10) were determined. Tertiary substrates ( 2, 8, 9) invariably gave lower k N 3 −/ k SOH values than secondary ones ( 1, 3, 4, 5, 7); opposite from what is expected on the basis of the stabilities of the respective free carbanium ions. This was explained by the attack of the nucleophile on ion pair(s) rather than on free carbonium ions. The magnitude of the competition factor seems to yield useful mechanistic information only in two extreme cases, i.e. direct displacement (S N2) and free stable carbonium ions (tertiary aromatic substrates-dissociative S N1 mechanism). In all other cases, especially with secondary substrates, the mechanistic evaluation of the k N 3 −/ k SOH values is ambiguous and in our opinion of doubtful value.

Cationic polymerization of cyclic dienes. X. Cationic polymerization of 1‐vinylcyclohexene and 3‐methyl‐1,3‐pentadiene

Abstract1‐Vinylcyclohexene (VCH), which has one of the double bonds in the ring and the other outside the ring, was synthesized and polymerized by cationic catalysts. The reactivity of VCH was very large in the polymerizations catalyzed by boron trifluoride etherate (BF3OEt2) and stannic chloride–trichloroacetic acid complex. Similar to other cyclic dienes, the polymerization of VCH was a nonstationary reaction having a very fast initiation step. The polymerization proceeded by either a 1,2‐ or a 1,4‐propagation mode in which vinyl group was always involved. Particularly when BF3OEt2 was used as a catalyst, an intramolecular proton or an intramolecular hydride ion transfer reaction took place, resulting in the formation of methyl groups in the polymer. The degree of polymerization of polymer formed was about 10. This indicates the preponderance of monomer transfer reaction. To investigate the reason for the high reactivity of cyclic dienes, cationic copolymerizations of VCH and 3‐methyl‐cis/trans‐1,3‐pentadiene (cis/trans‐MPD) was carried out. The relative reactivity of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. On the other hand, the resonance stabilization of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. Therefore, it could be considered that the monomer reactivity is mainly determined by the stability of carbonium ion intermediate. The relative stability of carbonium ion must be VCH > trans‐MPD > cis‐MPD. Thus the influence of the conformation of ion on its stability was clearly demonstrated.

Desorption of carbonium ions from zeolite surfaces; A comparison of reactive surface ionization with field ionization

In comparison with field ionization on platinum the thermal desorption of surface carbonium ions from zeolite surfaces has been investigated mass spectrometrically. CaY-zeolites — forming carbonium ions — were supported by Pt filaments or Pt emitter tips where hydrocarbons are chemisorbed in a non-ionic form. Thermal ion desorption at very low external fields has been observed with stable carbonium ions of triphenylmethyl compounds Ph 3 C-X. This ion desorption was achieved on CaY zeolite surfaces and occurred in the order X  Br > X  Cl > X  OH; however, no ions could be obtained with X  H and X  COOH up to 825°K. On Pt surfaces high fields were required for desorbing molecular ions. The ion desorption from the zeolite and the field desorption from platinum could be distinguished by an energy analysis of desorbed ions. Those ions originating from the dielectric grains of CaY-zeolite suffer field dependent energy losses and therefore a virtual shift in the mass scale. Thermal desorption (a) and field desorption (b) differ in principle: (1) in the kind of ions formed, (a) carbonium ions from zeolite surfaces, (b) parent molecular ions from platinum; (2) in the temperature dependence of ion currents, in (a) a positive temperature function with necessary minimum temperature, in (b) no minimum temperature, and negative temperature coefficients have been observed; (3) in the field dependence of ion currents (a) very low fields < 10 5 V cm were sufficient, and Schottky-line dependences of intensities have been found, in (b) a minimum field > 10 7 V cm was required. Ion formation on zeolite surfaces is facilitated by an intra-molecular substitution of Ph 3CX, which polarizes the C δ+−X δ- bond, and as well by an intermolecular interaction of electron acceptors, for instance by [(CN) 2-C  C-(CN) 2]. Experimental results are discussed with respect of surface fields on zeolite structures, Brönsted- or Lewis acid sites, and the formation of charge-transfer complexes. In conclusion, acidic surface sites are most likely responsible for the carbonium ion desorption in this investigation.

Nuclear magnetic resonance method for determination of aryl carbonium ion stabilities (.DELTA.FoR+). Factors which influence stabilities

Nuclear magnetic resonance method for determination of aryl carbonium ion stabilities (.DELTA.FoR+). Factors which influence stabilities

Cationic polymerization of indene

Cationic polymerization of indene in methylene dichloride was effected by two stable carbonium ion salt initiators, tropylium hexachloroantimonate and xanthylium perchlorate, the former being more efficient. Very high conversions of monomer to polyindene were obtained with catalyst concentrations ∼10 −2 M. Molecular weights were approximately 10 000 and it is concluded that whilst initiation is fairly rapid and complete, active centres are lost by creation of more stable cationic species from polyindene chains. Mechanisms are proposed to account for the formation, during polymerization, of characteristically coloured species.

The electronic structure of carbonium ions

A large number of carbonium ions are calculated with a modified CNDO procedure. The study of the electronic structure of these electron deficient compounds using energy partitioning and localized orbital techniques leads to an understanding of the observed trends of the stabilities of carbonium ions. The stabilization of a positive charge by methyl substituents is broken down into hyperconjugative, hybridization and inductive effects. Nonclassical structures characterized by three center bonds are also investigated. The chemical reactivity of carbonium ions is explained by the ability of the empty orbital to combine steadily with bond orbitals.

The formation of carbonium ions from arylmethanes on silica-alumina

The chemisorption of triphenylmethane, diphenylmethane, and 4,4′-dimethoxy-diphenylmethane on silica-alumina cracking catalysts was studied in vacuo and in the dark. The substrate reacted on the surface forming stable carbonium ions, e.g., the triphenylcarbonium ion from triphenylmethane. The fate of the H − ion which is stoichiometrically lost concomitant with ion formation was ascertained by extracting and analyzing the products of the surface reaction. With triphenylmethane the results were found to conform to the following Friedel-Craft's chemistry: ▪ Surface densities of triphenylcarbonium ions, determined by recovery as triphenyl-carbinol, agreed with those determined spectrophotometrically (~5 × 10 11/cm 2). With diphenylmethane the primary reaction involved alkylation, i.e., ▪ The reaction of 4,4′-dimethoxydiphenylmethane followed both reaction paths, but the alkylation products were too complicated to characterize. It was concluded that the formation of the triphenylcarbonium ion on these catalysts does not demonstrate that hydride ions are abstracted from paraffinic carbon atoms by the silica-alumina surface as previously supposed.

Stable carbonium ions. CVII. Diprotonated hydroxycarboxylic acids and their cleavage in fluorosulfuric acid-antimony pentafluoride solution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStable carbonium ions. CVII. Diprotonated hydroxycarboxylic acids and their cleavage in fluorosulfuric acid-antimony pentafluoride solutionGeorge A. Olah and Alice T. KuCite this: J. Org. Chem. 1970, 35, 11, 3913–3916Publication Date (Print):November 1, 1970Publication History Published online1 May 2002Published inissue 1 November 1970https://doi.org/10.1021/jo00836a072RIGHTS & PERMISSIONSArticle Views60Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (422 KB) Get e-Alerts Get e-Alerts

Stable carbonium ions. CXVII. Protonation of sulfites and sulfates and their cleavage reactions in fluorosulfuric acid-antimony pentafluoride solution

Stable carbonium ions. CXVII. Protonation of sulfites and sulfates and their cleavage reactions in fluorosulfuric acid-antimony pentafluoride solution

Stable carbonium ions. CXVI. Sulfonyl halide-antimony pentafluoride complexes and study of protonation and cleavage of sulfonyl halides in fluorosulfuric acid-antimony pentafluoride-sulfuryl chloride fluoride solution

Stable carbonium ions. CXVI. Sulfonyl halide-antimony pentafluoride complexes and study of protonation and cleavage of sulfonyl halides in fluorosulfuric acid-antimony pentafluoride-sulfuryl chloride fluoride solution